1. Field of the Invention
The present invention relates to a shadow mask for a color CRT (Cathode Ray Tube) and its fabricating method and, specially, to such a shadow mask and its fabricating method which is designed to have an ideal texture suitable for etching a shadow mask thin plate.
2. Discussion of Related Art
Color CRTs have been developed and are now being in need for the reproducibility of definite colors and in compliance with the developed transmission modes wherein the number of scanning lines is increased.
As shown in FIG. 1, a color CRT has panel 1 which is coated with fluorescent film 3 on the inner surface and coupled to funnel 2 with fused adhesive glass, the inner surface of the funnel 2 being coated with conductive graphite. Electron guns 6 to generate electron beams 5 are mounted in the neck 4 of the funnel 2, and shadow mask 7 functioning as color-discreted electrodes is supported by frame 8 in the panel 1. The circumferential surface of the funnel 2 has deflection yoke 9 mounted to deflect the electron beams 5 in the transverse direction.
The color CRT further includes an inner shield fastened to the frame 8 in order to prevent the deviation of electron beams 5 from their given paths due to the earth magnetic field or the leaked magnetic field.
Under the picture signals applied to electron guns 6 in the color CRT as constructed above, thermoelectrons released from the cathodes of the electron guns are accelerated and converged upon the panel 1 by means of the voltage applied to each electrodes of the electron guns 6.
At this stage, the electron beams 5 are adjusted in direction by the magnetic field of the deflection yoke 9 built in the neck 4 of the funnel 2, the beams 5 being affected by the inner shield so as not to deviate from the given paths and projected onto the whole surface of the panel 1. Passing through the slots of the shadow mask 7 associated with the frame 8 in the panel 1, the deflected electron beams 5 are divided by colors and strike the fluorescent film 3 coated on the inner surface of the panel 1, emitting lights to represent picture signals.
In the structure, the shadow mask 7 is made of rimmed steel (series by JISG 3141) or AK (Aluminum Killed) steel.
As the color CRTs have been developed to be high definition, however, these materials have a high heat expansion coefficient of about 11.5.times.10.sup.-6 deg.sup.-1 that the electrons given off by the electron beams strike the shadow mask, producing heat sufficient to expand the shadow mask. This causes a phenomenon, so-called doming effect, where the electron beams cannot reach designated spots on the fluoresent film but strike unwanted colors. Doming effect has become an important problem with televisions, displays and the like which have been developed to be of high definition and high luminance.
A previous method involves the use of material having a low heat expansion property of the shadow mask for the purpose of prevention of the doming effect. For example, the shadow mask may be made of Fe--Ni invar-based alloy (Ni: 36%, Fe: 64%) having the low heat expansion coefficient of 1.5.times.10.sup.-6 deg.sup.-1, which method is described in JP-A 61-78033 and JP-A 04-56107.
To prepare a shadow mask, a thin invar disc plate is perforated with several hundred and thousands or millions of holes for the electron beams to pass through, in an etching step and undergoes a heat treatment and a molding step to have the curved profile.
The shadow mask is manufactured from a thin plate made of invar alloy and having the thickness between 0.1 and 0.2 mm, which is removed fat, washed, coated with a photoresist, exposed, developed, etched, removed the photoresist, cut, etching again and annealed, followed by a press forming, blackening, welding and packing.
While the heat expansion coefficient of the shadow mask made of AK steel is 11.5.times.10.sup.-6 deg.sup.-1, the invar shadow mask has a low heat expansion coefficient (1.5.times.10.sup.-6 deg.sup.-1) so as to form of hole for the electron beams to pass through at adequate positions in the shadow mask irrespective of temperature. Invar-based material is expected to be used in manufacturing the shadow mask suitable for the display of a computer that requires high definition still picture for high definition and high quality television broadcast, but required to be improved in etching and forming properties.
For high quality shadow mask, it is necessary to perforate the shadow mask with hole having small pitch and uniform profile by the etching step.
Furthermore, the plate made of Fe--Ni alloy is hard to have an etching in making the hole of uniform profile compared with the AK steel plate.
For example, this problem of etching with the Fe--Ni alloy shadow mask can be improved by controlling the carbon content below 0.01% (JP-A 61-82453) or by limiting the non-metallic inclusion (JP-A 61-84356).
JP-A 59-32859, JP-A 61-19737, KR88-102, U.S. Pat. No. 4,528,246 and KR97-147 disclose the use of invar shadow mask which is manufactured to have the {100} texture of 35% or more by controlling the cooling and annealing steps in the manufacture of the invar shadow mask material.
In the cited references, it is described that such an invar shadow mask material is easily perforated with the hole of uniform size by an etching step thereby improving the doming property with high reproducibility of definite colors.
However, the material having the carbon content even below 0.01% contains contaminating substances such as S, B and N which are segregated in the grains or enter the crystals as interstitial atoms and affect the etching property of the material.
If the {100} texture is developed, the etching is improved because a {100} crystal plane has the highest etching rate. However, when the {100} texture has a high degree of integration, particularly above 95%, the etching is performed so fast along the crystal lattice as to form non-uniform hole without a roundness. Therefore, it cannot be said that the optimum crystal orientations for an adequate etching are obtained by the {100} texture of more than 35% as suggested in JP-A 61-19737.